As rare earth-doped fiber amplifiers are being used in new applications such as multiple-wavelength WDM systems and analog CATV systems, spectral gain flatness is becoming increasingly important. While silica-based erbium-doped fiber amplifiers (EDFAs) can produce high output power and low noise figure (NF) when pumped near 980 nm, the usable gain bandwidth for optical communications is limited to about 12 nm because the gain is spectrum is not flat and varies with operating conditions (signal power, etc.). Improved gain flatness has been achieved using an erbium-doped fluoride-based fiber amplifier (EDFFA). However, such amplifiers are more difficult to produce, handle and splice, have not yet achieved high output powers and can only be pumped near 1480 nm, which limits the lowest achievable noise figure to about 4 dB.
Gain flatness has also been improved by means of optical filtering (see R. A Betts et al., Technical Digest OFC '95, Paper TuP4) or by combining fibers of different host composition (see T. Kashiwada et al., Technical Digest OFC '95, Paper TuP1). US patents 5,131,069, 5,216,728, 5,260,823 and 5,583,689 also pertain to filtered fiber amplifiers. U.S. Pat. No. 5,731,892 pertains to multistage fiber amplifiers.
Prior art fiber amplifiers typically can provide substantially flat gain over a spectral region of width up to about 30 nm. See, for instance. U.S. Pat. No. 5,260,823. This limits, for instance, the number of communication channels that can be transmitted through a single mode optical fiber communication system. It clearly would be desirable to have available an optical fiber amplifier that can provide substantially flat gain over a wider spectral region, exemplarily 40 nm or even more. Furthermore, it would be desirable if the amplifier utilized silica-based optical fiber, thereby avoiding compatibility problems with silica-based transmission fiber and other silica-based fiber components. Still furthermore, it would be desirable that the increased bandwidth be attainable without significant degradation of the amplifier noise figure (NF) and without substantial reduction in output power. This application discloses such an amplifier, and optical fiber communication systems that comprise the amplifier.
Using splitting of the optical channels into two bands and gain equalization filters, Yan Sun et al. recently were able to achieve relatively flat gain over 80 nm bandwidth. See Yan Sun et al., "Optical Amplifiers and Their Applications," 1997 Technical Digest, pp. 144-147, Optical Society of America. However, the approach is complex and thus expensive, requiring inter alia circulators, isolators and a considerable quantity (e.g., 200-500 m) of Er-doped silica fiber. Clearly, it would be desirable to have available a less complex technique for achieving flat gain over a large bandwidth, e.g., 40 nm or more, even 80 nm.